1. Field of the Invention
Embodiments of the present invention relate generally to power conversion circuits and, more specifically, to a non-linear compensation ramp for current mode pulse width modulation.
2. Description of the Related Art
Electronic systems commonly employ power conversion subsystems to provide regulated voltage supplies to various circuits requiring stable supply voltages. One type of power conversion subsystem is referred to as a direct current-to-direct current (DC-to-DC) regulator. DC-to-DC regulators are typically configured to periodically transmit energy from an energy source, such as a battery, to an energy load that requires a specific output voltage or current. DC-to-DC regulators conventionally employ a technique known in the art as pulse width modulation (PWM) in which an amount of energy proportional to a pulse width is transferred through an electronic switch from the energy source to the energy load in order to maintain the specific output voltage or current, even as the energy source and energy load vary. Regulation is achieved by modulating the pulse width appropriately to adjust how much energy is transferred in each period to compensate for changing source and load conditions.
A DC-to-DC voltage regulator configured to implement PWM commonly comprises an inductor, capacitors, a diode, external or internal MOS, and at least one electronic switch MOS configured to periodically transfer energy from the inductor to the capacitor, based on a feedback system that samples the output voltage and current in the electronic switch. If the output voltage trends lower during a given period, a proportionally larger amount of energy is transferred from the inductor to the capacitor in a subsequent energy transfer period. Similarly, if the output voltage trends higher during a given period, a proportionally smaller amount of energy is transferred from the inductor to the capacitor in a subsequent energy transfer period. The feedback system may use sampled voltages for feedback or a combination of sampled voltages and sampled currents for feedback.
A current mode DC-to-DC voltage regulator includes a feedback system that uses both sampled voltages and sampled currents. Voltage sampling compares the output voltage with a reference voltage to generate an error voltage that is proportional to the difference between the output voltage and reference voltage. Current sampling compares the instantaneous current flowing in the inductor with the error voltage to determine a duty-cycle for the power switch. The voltage sample feedback path comprises an outer feedback control loop, while the current sample feedback path comprises an inner feedback control loop.
Current mode DC-to-DC voltage regulators typically exhibit greater regulation load regulation stability, which is a desirable characteristic. However, under certain conditions (Switch Duty>50%), current mode DC-to-DC voltage regulators exhibit instability that must be managed. One example of instability is known in the art as sub-harmonic oscillation, where the outer feedback control loop and inner feedback control loop generate opposing feedback responses in each period, creating a lower frequency (sub-harmonic) oscillation. This sub-harmonic oscillation signal is emitted from the regulator as noise in the output voltage.
A technique known in the art as slope compensation may be used to reduce the sub-harmonic oscillation. Slope compensation uses a summation circuit to add a linear ramp function to the sampled version of a current ramp in the inductor. The sum signal rather than the inductor current signal is then used as the feedback signal for the inner feedback control loop. Adding in the linear ramp function has the effect of lowering the current feedback loop as a function of on time (% duty cycle) for the electronic switch. However, without slope compensation, when the electronic switch duty cycle is operating at greater than 50%, small perturbations in the feedback system may be amplified in sequential cycles, causing unstable operation of the regulator.
As the foregoing illustrates, what is needed in the art is an approach for controlling current mode DC-to-DC voltage regulators that is more robust over a wider range of operating conditions than is provide for in existing art.